1. Field of the Invention
The present invention relates to a brushless motor and, more particularly, to a small and thin brushless motor for rotating a signal recording/reproducing apparatus. The invention also relates to a recording disk driving apparatus having the brushless motor.
2. Background Art
In recent years, as a hard disk drive (hereinbelow, called HDD) for recording/reproducing a magnetic disk as a signal recording/reproducing apparatus is becoming smaller, thinner, and more portable, reduction in the size, thickness, and current of a brushless motor mounted on the HDD is in demand. To meet the demand, an inner rotor type brushless motor in which a rotor magnet is disposed on the inner circumference side of a stator is being employed in recent years.
However, as the size and thickness of the inner rotor type brushless motor is further reduced, the following problems may occur. When the size and thickness of the whole brushless motor is reduced, a clearance in the axial direction between the rotor magnet and a base plate for holding a stator is also reduced. For example, when the base plate is made of a ferromagnetic material, there is a case such that part of the magnetic flux of the rotor magnet flows not to the stator side but to the magnetic base plate side and the torque decreases. In other words, a magnetic path is formed between the rotor magnet and the magnetic base plate, so that an effective magnetic path between the rotor magnet and the stator decreases, sufficient torque of the brushless motor cannot be obtained, and a start trouble such that the torque necessary at the time of starting the brushless motor is insufficient and the brushless motor does not start may occur.